In a packet-switched communication network, data are transmitted in the form of packets that are routed from a source node to a destination node through possible intermediate nodes. Exemplary packet-switched networks are IP (Internet Protocol) networks, Ethernet networks and MPLS (Multi-Protocol Label Switching) networks.
Packets not always reach the destination node, i.e. they may be lost during transmission through the network. Loss of packets is due to different reasons. For instance, a node or link may fail, or packets may be discarded by a node due to a congestion of its ports. Besides, packets may be discarded by a node since they contain bit errors. In any case, when providing a service by transmitting data through a packet-switched network, the rate of packets lost during transmission affects the quality of service (QoS) of that service.
Besides, a packet is transmitted at a transmission time by the source node and is received at a reception time by the destination node. The time elapsing between transmission time and reception time is typically called “one-way delay”. The one-way delay of a packet mainly depends on the number of possible intermediate nodes crossed by the packet from source to destination, the permanence time of the packet at each node and the propagation time along the links. Since the packets are routed hop-by-hop by each node, both the number of possible intermediate nodes crossed by packets and the permanence time of packets at each node are unpredictable. Accordingly, the one-way delay of a packet is almost unpredictable.
Besides, packets of a same packet flow may have different one-way delays. The difference between the one-way delays of two packets of a same data flow is termed “interarrival jitter” (or, briefly, “jitter”).
When a communication service (in particular, a real-time voice or data service such as call, conference call, video conference, etc.) is provided by means of a packet-switched network, a measurement of packet loss, one-way delay and jitter affecting the packet flow carrying the service provides an indication of the quality of service (QoS) perceived by the end users of the service. Therefore, measuring packet loss, one-way delay and/or jitter of packet flows in a communication network is of particular interest for network operators.
WO 2010/072251 (in the name of the same Applicant) discloses a method for measuring data loss of a data flow transmitted through a communication network from a transmitting node to a receiving node. Before transmitting the data units of the data flow, the transmitting node marks each data unit for dividing the data flow in blocks. In particular, the transmitting node marks each data unit by setting a bit of its header to “1” or “0”. The marking results in a sequence of blocks, where blocks of data units marked with “1” alternate in time with blocks of data units marked with “0”. The blocks may have a same duration termed “block period” Tb (e.g. 5 minutes).
WO 2011/079857 (in the name of the same Applicant) discloses a method for performing a time measurement (in particular, for measuring one-way delay and/or jitter) on a data flow. According to WO 2011/079857, in addition to the above marking operation disclosed by WO 2010/072251, transmission and reception timestamps are generated at each block period, which indicate the times at which predetermined data units of the current block are transmitted and received. In particular, the transmitting node counts the packets being transmitted during the current block period and generates a transmission timestamp every X transmitted packets. The receiving node, similarly, counts the packets being received during the current block period and generates a reception timestamp every X received packets. At the subsequent block period, the transmission and reception timestamps are used for calculating the one-way delay and jitter affecting the predefined data units.
The Internet draft “Coloring based IP flow performance measurement framework, draft-chen-coloring-based-ipfpm-framework-01”, Feb. 25, 2013 discloses to set an unused bit of the IP header to “color” the packets into different color blocks to enable packet loss and delay measurements. For delay measurements, within a time period a single packet is colored. The sender records the timestamp when the colored packet is transmitted, the receiver records the timestamp when detecting the colored packet. With the two timestamps, the packet delay is computed.